1. Field of the Invention
The invention relates generally to methods of converting urea to ammonia. In particular, the invention relates to quantitative conversion of urea to ammonia in response to a demand for ammonia. More specifically, the invention relates to quantitatively converting urea to ammonia at a rate closely following the instant demand rate for ammonia, for example in a system for quantitative chemical treatment applications, such as removal of oxides of nitrogen from or conditioning fly ash present in fuel combustion tailgas streams.
2. Brief Description of Related Technology
Ammonia has long been known to be useful in the treatment of stack gases (also referred to herein as flue gases or tail gases) from fossil fuel combustion processes, for example, coal-fired electric power generating plants. The boiler effluent gas stream from such a process contains oxides of nitrogen (NOx) that are well known air pollutants, contributing to photochemical smog and causing other deleterious consequences to the environment. Under the right conditions, ammonia will react with NOx, converting the nitrogen oxides to nitrogen and water. The most familiar methods for NOx destruction using ammonia are known as Selective Catalytic Reduction (SCR) and-Selective Non-Catalytic Reduction (SNCR). Of these, the SCR methods achieve the highest removal efficiency. The boiler flue gases from electric generating plants also contain fly ash, which is often collected by means of electrostatic precipitators. Ammonia conditioning (controlled injection into the flue gas stream) is often beneficial in such systems to improve fly ash collection efficiency and improve performance. In both of these applications, the treatment process requires precise quantitative delivery of ammonia at a rate that follows a variable demand.
Ammonia may be obtained as an anhydrous liquid or as an aqueous solution, but in either case the safety issues are quite severe. The daily quantities of ammonia required to support a large industrial application may be very substantial, necessitating large capacity storage facilities at the site. Ammonia is a dangerous chemical with a noxious odor, it is very toxic to most life forms, highly volatile, and is also potentially explosive. Numerous regulations apply to the safe transport, storage and handling of ammonia. Storage facilities require containment and deluge systems, continuous safety monitoring, periodic inspections, specialized training programs, and special operating permits. Ammonia is classified as a hazardous material, and storage of large quantities of such a chemical in many locations, for example, near urban population centers, is highly undesirable.
In contrast to ammonia, urea is an innocuous raw material, which may be hydrolyzed to form ammonia and carbon dioxide gases. Several urea hydrolysis processes have been described with the intent to generate a gaseous stream of ammonia, carbon dioxide, and water vapor at a temperature and pressure useful for removal of nitrogen oxides, treatment of fly ash or for other compatible process applications.
Young U.S. Pat. No. 5,252,308 (Oct. 12, 1993), the disclosure of which is incorporated herein by reference, describes two variations of a process that performs urea hydrolysis in the presence of an acid. The Young patent discloses the use of certain protic mineral acids (e.g., hydrochloric acid) and polyprotic mineral acids, such as phosphoric or sulfuric acid, and monoammonium dihydrogen phosphate (also referred to herein as MAP). The Young patent teaches that the chemical reaction mechanism employs the acid and its monoammonium salt as intermediates, and that the reaction results in a quantitative production of ammonia from urea. The Young patent does not disclose how to control the apparatus to quantitatively produce a gaseous ammonia stream at a variable rate suitable for quantitative chemical treatment applications, such as removal of oxides of nitrogen from or conditioning fly ash present in fuel combustion tailgas streams.
Lagana U.S. Pat. No. 5,985,224 (Nov. 16, 1999), the disclosure of which is incorporated herein by reference, describes an apparatus that hydrolyzes an aqueous urea feed (from dissolved solid urea) in a heated and pressurized reactor vessel, and that uses steam to strip the ammonia and carbon dioxide product gases. The inlet urea solution flows through a series of divided chambers in the reactor, becoming gradually more dilute, and emerges from the reactor for recycle back to the urea dissolving portion of the system. The Lagana patent does not disclose the use of any compounds to enhance the rate of reaction, but does disclose steam stripping. The process according to the Lagana patent requires a large energy input for steam to heat the reactor contents and strip the product gases from solution. The Lagana patent does not disclose how to control the apparatus to quantitatively produce an ammonia stream at a variable rate suitable for quantitative chemical treatment applications, such as removal of oxides of nitrogen from or conditioning fly ash present in fuel combustion tailgas streams.
Cooper et al. U.S. Pat. No. 6,077,491 (Jun. 20, 2000), the disclosure of which is incorporated herein by reference, describes a process and apparatus for producing a gaseous ammonia stream from aqueous solutions of urea and/or biuret. In the methods disclosed and claimed in the Cooper et al. patent, the product gas flow rate is regulated in response to the demand for ammonia by the external process. Withdrawing gas from the reactor in this manner causes corresponding variation in the reactor pressure and temperature, which would necessarily change the rate of reaction. For example, when gas is released at an increased rate in response to an increase in demand, the pressure inside the reactor will drop, causing more water to flash to the vapor phase, which will lower the temperature of the reactants and slow the rate of urea conversion to ammonia. In addition, the concentration of ammonia in the product gas stream will drop in relation to the steady-state concentration that existed just prior to the change (e.g., increase) in demand and concomitant release of product gas. The disclosed control scheme compensates by varying the heat input to the reactor to raise the temperature (increasing the rate of reaction) which, in turn, will eventually restore the pressure. As the liquid level in the reactor begins to fall, a level control loop increases the flow of aqueous urea fed to the reactor to restore a balance at the higher production rate. By controlling the process in this manner, the rate of reaction and concentration of ammonia in the product gases (and, thus, the amount of ammonia delivered to the external use) are variable, even though the product stream mass flow rate is matched to the external demand for ammonia. The process must reach a full steady state before the ammonia production matches the demand. Large and sudden changes in ammonia demand will cause a significant upset in the process, and can require additional control means, such as a quench-cooling system to effect a rapid reduction in production. Restoration of a sudden large demand following such a quench-cooling event would require reheating the entire reactor and its contents, which requires a significant amount of time.
To successfully employ urea hydrolysis for quantitatively generating ammonia for use in applications for removal of NOx from stack gas streams using selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR) process technology (referred to herein as a deNOx system or a deNOx application), the rate of ammonia produced and discharged by the urea hydrolysis process must closely follow an ammonia demand signal. The ammonia demand rate signal can be a function of a boiler load, a measured ammonia slip (defined below), the efficiency of the reaction of ammonia with nitrous oxides, a combination of such measurements, or any other suitable measurement. The ammonia demand rate is the instantaneous requirement from the ammonia-consuming application that provides a moving target that a urea hydrolysis system attempts to satisfy.
Changes in the ammonia demand signal can be dramatic in real applications. For example, in an electric generating plant the boiler load alone can change as much as four times in magnitude (25% to 100% for example) over the course of a typical 24-hour operating day. A urea hydrolysis process must be able to quickly adjust to large changes in ammonia demand, and the quicker the response time, the better the urea hydrolysis process. It is also important to consider the effect of a sudden increase in demand as compared to a sudden decrease in demand. With a sudden increase in demand, the ammonia generation system (urea hydrolysis system) will be increasing the ammonia production, and therefore any lag results in a temporary under-feeding to the deNOx system. The consequence is a temporary increase in NOx concentration in the stack gases (assuming ammonia is properly distributed, etc.). With a sudden decrease in demand, a more serious situation can result. In this case, as the ammonia generator (urea hydrolyzer) reduces the ammonia production to the new, lower, requirement, a transitory over-feeding of ammonia can occur. If excess ammonia is supplied (assuming good distribution), then some residual ammonia can pass through the deNOx system unreacted and go up the stack to the atmosphere. The presence of unreacted ammonia vapor in the stack discharge is referred to as “ammonia slip,” and it is a performance measure that has governmental regulatory significance, since ammonia is a poisonous and obnoxious-smelling gas. Typical performance guarantees on deNOx process systems are in the range 1 ppm to 3 ppm of ammonia slip.
In a commercial scale deNOx system the rate of ammonia consumption is quite large. Therefore, it would be very costly (impractical) to provide a large “surge tank” (or pressurized storage reservoir) for ammonia and carbon dioxide gases and water vapor produced in a urea hydrolysis reaction, which gases would also have to be maintained at a temperature above 60° C. to avoid formation of ammonium carbonate, and from which a flow of these gases could be withdrawn to meet the instant demand. Instead, it would be desirable to provide a urea hydrolysis process and apparatus that enables a simple fast-acting control to produce ammonia in a reactor at a rate equivalent to the instantaneous demand of an ammonia-consuming process, for example.
Accordingly, it would be desirable to have an energy-efficient process and apparatus for converting urea to ammonia and producing an ammonia-containing product stream at a rate that can be quickly changed in response to a demand for quantitative chemical treatment applications.